1. Field of the Invention
The present invention is directed to a method for echo compensation and to an arrangement for the implementation of this method.
2. Description of the Related Art
Due to the finite attenuation of hybrid circuits, an echo signal that stems from a remote subscriber station is superimposed on the transmission signal of a first subscriber station. A correction signal for the compensation of the echo signal is determined from the received signal in an echo compensator with the assistance of an FIR filter (with finite pulse response) that is sometimes supplemented by a recursive filter. It is standard to process digitalized signals in the echo compensators. The realization can ensue both as a traditional hardware circuit with memories, multipliers and adders or, on the other hand, as a software solution with signal processors.
For reducing the circuit or, respectively, calculating outlay, it is known to divide the signals to be processed into sub-bands.
German letters Patent DE 689 12 372 T2 discloses a means for echo compensation with filtering in frequency sub-bands that is utilized for filtering interference caused by acoustic couplings. A plurality of parallel processing channels that respectively comprise two analysis filters and a symmretrical synthesis filter are provided for echo suppression.
Published European Patent Application EP 0 615 350 A1 discloses a method for the compensation of acoustic noise signals wherein the signal to be compensated is divided into a plurality of sub-bands that are likewise individually compensated. The plurality of filter coefficients of the adaptive filters is controlled for further reduction of the convergence time.
An object of the present invention is to reduce the outlay even farther.
This object is achieved by the method for echo compensation wherein the signal to be compensated and a received signal serving for the determination of correction signals are respectively divided into a plurality of mutually corresponding sub-bands, and a compensated signal is acquired in each sub-band by setting the filter coefficients of compensator filters, a compensation signal in at least one sub-band is acquired in a filter arrangement composed of a series circuit of an adjustable delay element and a compensator filter shortened by roughly the running time thereof, being acquired by setting the delay time of the delay element and the filter coefficients of the compensator filter.
It is thereby advantageous that a full-length filter is only required for one sub-band. A series circuit of a variable delay element and a shortened filter is respectively employed for the other sub-bands. As a result thereof, fewer multipliers or, respectively, fewer calculating steps are required, since the calculating performance is proportional to the plurality of filter coefficients.
Given more than two sub-bands, the delay elements of the remaining filters can be set to the same delay time, except for the first sub-band for which a full-length filter is required.
The calculating outlay or, respectively, circuit outlay drops approximately inversely with the number of sub-bands since fewer coefficients need be calculated in the individual filters and lower processing frequencies can be employed.
The echo compensator is also especially suited for multi-tone reception devices; fundamentally, however, it is not limited thereto. The sub-bands can be re-combined into one band with a synthesis filter bank.